So how do you tell which organisms on a tree are most closely related to one another? It’s tempting to focus on the order of the branch tips on a tree (i.e., which lineage goes to the right and which goes to the left), but in fact, this ordering is not meaningful at all. Instead, the key to understanding evolutionary relationships is common ancestry. Common ancestry refers to the fact that distinct descendent lineages have the same ancestral lineage in common with one another, as shown in the diagram below.

Any set of taxa has a most recent common ancestor — that is, the youngest common ancestor that they all have in common. To find the most recent common ancestor of a set of taxa on a phylogenetic tree, follow each taxon’s lineage back in time (towards the base of the tree) until all the lineages meet up. That node represents their most recent common ancestor. Test your understanding of most recent common ancestors with the tree shown here. Which node represents the most recent common ancestor of the square taxon and the star taxon? Click the button to see the answer.

Taxa that share a more recent common ancestor with one another are more closely related than are taxa whose most recent common ancestor is older. As shown in the diagram above, because the triangle taxon shares a more recent common ancestor with the square taxon than either does with the star taxon, we can say that the triangle and square taxa are more closely related to one another than either is to the star taxon.

Test your understanding of evolutionary relationships with the phylogenetic tree shown here. Which taxa are more closely related, the oval and the triangle or the triangle and the star? Click the button to see the answer.